This is a proposal 1) to continue follow-up cohorts of patients with multiple sclerosis (MS), one in King and Pierce Counties, Washington, and one in Los Angeles County, California, that included individuals in all stages of disease in 1970 and 2) to carry out surveillance for new cases of MS 17-22 years after the introduction of measles vaccine. The objective of the study are a) to determine if the clinical course of disease is related to place of residence in a high- or low-prevalence area, onset symptoms, early course, age at onset, sex and other factors, b) to determine if the introduction and widespread use of attenuated measles vaccine and the concurrent dramatic drop in infection with wild measles virus results in a change in the incidence of MS after an interval compatible with the induction period hypothesized from epidemiologic studies and c) to determine if the entire course of MS from initiation of the pathologic process to completion of clinical course (induction period plus clinical course) is more rapid and severe in 10 prevalence areas as suggested by 8 years of follow-up of the two cohorts.